I'm building (actually finishing an upstairs room) theater room, and it won't be dedicated to HT only. I was planning on drywall as the wall and ceiling material but read somewhere, Crutchfield I think, that drywall absorbs a lot of sound leaving the bass weak among other things. The recommendation was to install plywood and then the drywall over that.
How concerned should I be with this and is it worth the extra money to install the plywood and teh drywall? Can OSB be used instead of plywood?
Thanks.

Use drywall. Plywood IMO has one use - to allow you to put screws anywhere instead of finding studs. Drywall is cheaper and provides more mass for isolation purposes. Believe me, a room with a double layer of drywall will do anything but suck up all the bass.

Bryan